This invention is directed to a gas-tight, sealed metal oxide/hydrogen storage battery with a positive metal oxide electrode, a negative hydrogen electrode, a separator arranged between the positive and negative electrodes and containing an alkaline electrolyte, and means for the catalytic recombination of oxygen developed at the positive electrode in the event of overcharging.
Well-known among secondary cells of this general type are metal oxide/hydrogen storage batteries having negative electrodes synthesized from alloys of the La/Ni or Ti/Ni systems. These alloys also frequently contain V, Cr, Zr, Mn, Al, Co, and the like, as secondary components.
As in the case of gas-tight nickel/cadmium storage batteries, the gas-tight operation of these cells requires continuous elimination of the oxygen which is developed at the positive electrode in the event of overcharging. This is because, due to the lower output capacity of the positive electrode, the production of oxygen generally takes place as a result of direct electrochemical reduction at the negative electrode, i.e., the cells operate "in an oxygen cycle".
However, unlike the nickel/cadmium storage battery, for the cells being considered here, the developing oxygen must be transferred to a negative electrode which is a hydrogen-storing metal hydride. Therefore, it must be assumed that on this electrode, both a chemical reaction of oxygen with the stored hydrogen (H.sub.st) will occur (Equation 1), and an electrochemical reduction of oxygen will take place on the electrode surface (Equation 2): EQU 4H.sub.st +O.sub.2 =2H.sub.2 O (1) EQU 2H.sub.2 O+O.sub.2 +4e=4OH (2)
However, in addition to these processes, which are not critical to the operability of the active mass, a parasitic side reaction is possible which can adversely affect the electrochemical properties of the active mass. In this side reaction, oxygen coming from the positive electrode can form oxides with the components of the active mass (alloy), as follows: EQU 2.times.Me+O.sub.2 =2Me.sub.x O
Since these oxides tend to develop on the surface of the metal hydride particles, such oxides can considerably hinder the dynamics of hydrogen take-up and emission when charging and discharging. Consequently, the negative electrodes of metal oxide-hydrogen storage cells are exposed to the risk of corrosion.
In order to prevent this, it is suggested in DE-PS 28 38 857 to allow the oxygen consumption reaction to occur on an auxiliary electrode which is only in electron-conducting contact with the alloy electrode(s). For this purpose, an electrode arrangement is disclosed in which an auxiliary electrode is located between two positive electrodes in the electrode stack or group, and a first separator is provided which surrounds the auxiliary electrode, and which by means of its highly porous and hydrophobic nature promotes the flow of oxygen to the auxiliary electrode, while a second hydrophilic separator of lower gas permeability is positioned to separate the positive electrode from the negative electrode, to hinder the access of oxygen to the latter structure. However, the requirement of two different separator materials, and their arrangement between the plates for proper operation, requires special attention during assembly and tends to make the construction of such a storage battery rather complicated.